RFID labels, inlays, straps, and transponders (commonly referred to herein as “tags”) are widely used to associate an object with an identification code. Tags generally combine one or more antennas with an analog and/or digital electronic circuit chip (RFID chip) that may include, for example, communications electronics, data memory and control logic. Examples of RFID tag applications are automobile security-locks, access control to buildings, inventorying and parcel tracking. In general, RFID tags can retain and transmit enough information to uniquely identify individuals, packages, inventory and the like.
There are three basic types of RFID tags. A passive tag is a beam powered device which rectifies energy required for operation from radio waves generated by a reader. For communication, the passive tag creates a change in reflectivity of the field which is reflected to and read by the reader. This is commonly referred to as continuous wave backscattering. A battery-powered semi-passive tag also receives and reflects radio waves from the reader; however a battery powers the tag independent of receiving power from the reader. An active tag, having an independent power supply, includes its own radio frequency source for transmission.
Passive backscatter tags use voltage multipliers to convert RF signals into DC power to power the chips circuitry. The range of a passive RFID tag is limited by its ability to convert low amplitude RF signals into sufficient DC power to power the tag's circuits.
RFID tags can also include sensors, such as vibration sensors, temperature sensors, and light sensors. As an example, a temperature logging RFID tag would periodically sample the temperature of its environment and save the measured temperature to its memory. The reader can later read out this record of temperatures as well as other information from the tag, such as its ID.
The reader, sometimes referred to as an interrogator, includes a transmitter to transmit RF signals to the tag and a receiver to receive tag modulated information. The transmitter and receiver can be combined as a transceiver. Communications between a reader and tag is defined by an air interface protocol, such as (without limitation):
(i) EPCglobal's EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz, version 1.1.0 (http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2—1—1—0-standard-20071017.pdf) (hereinafter referred to as the “UHF Gen2 standard”);
(ii) adaptations of the UHF Gen2 standard for operation at high frequency (“HF”), for example at 13.56 MHz; and
(iii) ISO/IEC 18000-6 Information technology—Radio frequency identification for item management—Part 6: Parameters for air interface communications at 860 MHz to 960 MHz, Amendment 1: Extension with Type C and update of Types A and B. Each of the above protocols is incorporated herein by reference for all purposes.
Communication protocols, such as these, may require that a passive tag operate a timing circuit or maintain a flag value during a brief lapse of received power. For example, the UHF Gen2 standard requires persistence for flags SL, S1, S2, and S3, but not S0. U.S. Pat. No. 6,942,155, assigned to Alien Technology Corporation (“Alien,” also the assignee to this invention) and incorporated by reference herein for all purposes, provides various teachings on persistent flags and nodes. Other or related techniques have been purportedly suggested by the following patents (each of which is incorporated by reference herein for all purposes):                (i) U.S. Pat. No. 7,259,654; and        (ii) U.S. Pat. No. 7,215,251.        
As discovered by Alien, persistent nodes suffer from a latent susceptibility to light, even ambient light. That is to say, exposure to light can dramatically decrease persistent time. During development of the inventions herein, the inventors recorded the results below from conventional tags:
Measured Persistence Per-UHF Gen2Results (seconds)sistentrequirement40 WSunIC ChipNode(seconds)DarkAmbientLightLightVendor AS10.5-5.02.12.00.380  0.050ChipS2>287.09.00.070<0.02 S3>277.07.90.050<0.02 SL>2100.06.30.040<0.02 Vendor BS10.5-5.02.72.00.130<0.02 ChipS2>241.330.00.990  0.370S3>233.830.01.900  0.510SL>238.820.00.800  0.320Opaque encapsulation of an integrated circuit chip can reduce this susceptibility, but that may not be desirable or economical.
As an ostensibly unrelated problem, conventional readers generally require complex anti-collision software in order to poll and identify tags when many tags are disposed in its RF field. However, even after identification of each tag, one is unable to respectively associate identification information to each tag. In other words, conventional readers only indicate that a plurality of tags were read, not that a specific tag contained specific data. An individual tag must be physically or electrically isolated (e.g., shielded) to provide desired granularity in information. Readers can bear further complexity by implementing range and bearing techniques as described in U.S. Patent Application Publication No. 2005/0237953 (which is incorporated by reference herein for all purposes), assigned to Alien, to unambiguously associate specific tags to received information.